The present invention relates to methods for producing several varieties of beta" alumina (beta double prime alumina). More particularly, the present invention relates to a method for growing high quality single crystals of beta" alumina from a homogeneous melt using a Czochralski type process or analogous procedure.
Beta" aluminas are a large family of rhombohedral compounds typified by the well known Na.sub.2 MgAl.sub.10 O.sub.17 type. Na may be regplaced by K, Rb, Cs. Mg is replaceable by other divalent cations such as Ni.sup.2+, Co.sup.2+, Cr.sup.2+, Fe.sup.2+ and others with a similar cation radius as Mg.sup.2+. It is also replaceable with 1/2Li.sup.+ (balanced by 101/2Al.sup.3+). Many of these compounds, in particular the K and Li varieties are not achievable by the older method of crystal growth from a NaAlO.sub.2 (or KAlO.sub.2, etc.) melt because the beta" variety has an upper temperature stability below the melt temperature, e.g. "Na.sub.2 Li.sub.1/2 Al.sub.101/2 O.sub.17 " is not stable above 1600.degree. C. and "K.sub.2 Li.sub.1/2 Al.sub.101/2 O.sub.17 " is unstable above appproximately 1300.degree. C.
Beta" aluminas have become one of the most widely investigated solid electrolytes. The remarkable ion exchange characteristics of the beta" aluminas have been of particular interest. Both divalent and trivalent cations have been found to diffuse rapidly into the beta" alumina structure to provide high conductivity solid electrolytes.
H.sub.3 O.sup.+, hydronium (and other species such as NO.sub.2 .sup.+) are also known to exchange with beta" aluminas. A particularly important one is the H.sub.3 O.sup.+ exchange which produces a proton conducting material (e.g. for fuel cells). H.sub.3 O.sup.+ exchange with the Na beta" causes cracking of the crystal but does not impair the potassium beta" crystal structure because of the size similarity of K.sup.+ and H.sub.3 O.sup.+.
The ability of beta aluminas to accept monovalent, divalent and trivalent cations is expected to allow the production of a wide variety of materials having interesting properties in addition to the already known fast ion transport characteristics of such cation doped beta" aluminas. For example, the ability of neodymium exchanged beta double prime alumina to exhibit interesting optical and laser properties was recently reported by M. Jansen, et al (M. Jansen, A. Alfrey, O. N. Stafsudd, D. L. Yang, B. Dunn and G. C. Farrington, OPT. LETTS. 9 (1984) 119).
The diffusional doping of beta" alumina with various cations such as neodymium involves two basic steps. First, it is necessary to prepare single crystals of beta" alumina. Then, the crystals are ion exchanged to replace the alkali ions with the desired replacement cation. In order for useful cation doped crystals to be prepared, it is essential that a method be provided for growing large optical quality beta" alumina single crystals. The ability to provide a method for growing high quality large beta" alumina crystals will ultimately determine the applications and usefulness of the devices based on cation exchanged beta" alumina.
Flux evaporation is the only technique which has been successfully used so far to grow reasonably large single crystals of sodium beta" alumina. This method involves volatilization of sodium from a Mg containing melt in the beta alumina-NaAlO.sub.2 composition to form magnesium stablized sodium beta" alumina. Neither lithium stabilized sodium beta" or any potassium beta" has been grown by this kind of technique.
Although the flux method yields crystals of suitable quality and size for use in testing optical behavior, there have been problems in utilizing the flux evaporation method to produce crystals on a large scale for widespread applications. Particularly, the crystals produced by flux volatilization are generally not uniform in size and have variable optical quality.
A particular problem is that the Mg beta" variety that has been grown is susceptible to topotactic intergrowths of narrow layers of beta', beta"' and beta.sup.IV varieties which are contiguous components in the Na-Mg-Al-O phase diagram. As addressed here, the Li variety has the advantage that beta"' and beta.sup.IV varieties (with 6 layer spinel block types) are unknown and do not form. In associated powder work, the Li variety is more easily produced phase pure than the Mg version, i.e. lacking topotactical intergrowth.
Skull melting is another crystal growth technique which has been used to prepare single crystals of sodium beta" alumina (see S. Chen, D. R. White, H. Sato, C. J. Sandburg and H. R. Harrison, Proceedings Of The Conference on High Temperature Solid Oxide Electrolytes, Brookhaven National Laboratory, F. J. Salzano, Ed. Vol. 2, 121 (1983). The crystals resulting from skull melting growth techniques are generally too small even for use in testing the optical properties of the crystals once they are doped with a desired cation. Again, the technique is not of use above the stability temperature of the particular beta" desired.
It is well known that laser quality crystals of large size can be grown from a homogeneous melt using the Czochralski process or an analogous procedure. Czochralski type processes have not been used to grow sodium beta" alumina crystals because it is well known that sodium beta" alumina melts incongruently. Accordingly, Czochralski methods have in the past been thought to be not feasible for use in growing beta" alumina crystals. The possibility of using mixed eutectic melts to produce beta" alumina is complicated because of the beta" alumina's ability to exchange many different ions. It was feared that in a Czochralski type process, the exchange of ions from the additives would lead to impurities and contamination of crystals pulled from the melt in a Czochralski type process.
There presently is a need to provide a process for preparing large high quality beta" alumina crystals having both uniform optical quality and uniform size. The development of such a method will provide a convenient source of beta" alumina crystals for use in experimentation and commercial applications.